1. Field of the Invention
The present invention is directed toward methods for separating hydrocarbon gas constituents to more efficiently and economically separate and recover both the light, gaseous hydrocarbons and the heavier hydrocarbon liquids. More particularly, the methods of the present invention more efficiently and more economically separate propane, propylene and heavier hydrocarbon liquids (and, if desired, ethane and ethylene) from any hydrocarbon gas stream, e.g., from natural gas or gases from refinery or petrochemical plants.
2. Description of the Background
In addition to methane, natural gas includes some heavier hydrocarbons and other impurities, e.g., carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. After compression and separation of these impurities, natural gas is further processed to separate and recover natural gas liquids (NGL). In fact, natural gas may include up to about fifty percent (50%) by volume of heavier hydrocarbons recovered as NGL. These heavier hydrocarbons must be separated from the methane to provide pipeline quality methane and recovered natural gas liquids. These valuable natural gas liquids comprise ethane, propane, butane and other heavier hydrocarbons. In addition to these NGL components, other gases, including hydrogen, ethylene and propylene, may be contained in gas streams from refinery or petrochemical plants.
Processes for separating hydrocarbon gas components are well known in the art. C. Collins, R. J. J. Chen and D. G. Elliot have provided an excellent, general review of NGL recovery methods in a paper presented at GasTech LNG/LPG Conference 84. This paper, entitled Trends in NGL Recovery for Natural and Associated Gases, was published by GasTech, Ltd. of Rickmansworth, England, in the transactions of the conference at pages 287-303. The pre-purified natural gas is treated by well known methods including absorption, refrigerated absorption, adsorption and condensation at cryogenic temperatures down to about -175.degree. F. Separation of the lower hydrocarbons is achieved in one or more distillation towers. The columns are often referred to as demethanizer or deethanizer columns. Processes employing a demethanizer column separate methane and other more volatile components from ethane and less volatile components in the purified gas stream. The methane fraction is recovered as a purified gas for pipeline delivery. The ethane and less volatile components, including propane, are recovered as natural gas liquids. In some applications, however, it is desirable to minimize the ethane content of the NGL. In those applications, ethane and more volatile components are separated from propane and less volatile components in a column generally known as a deethanizer column.
An NGL recovery plant design is highly dependent on the operating pressure of the distillation column. At medium to low pressures, i.e., 400 psia or lower, the recompression horsepower requirement will be so high that the process becomes uneconomical. However, at higher pressures the recovery level of hydrocarbon liquids will be significantly reduced due to the less favorable separation conditions, i.e., lower relative volatility inside the distillation column. Prior art methods have concentrated on operating the distillation column at higher pressures, i.e., 400 psia or higher while attempting to maintain high recovery of liquid hydrocarbons. In order to achieve these goals, some systems have included two towers, one operated at higher pressure and one at lower pressure.
Many patents have been directed to methods for improving this separation technology. For example, see U.S. Pat. No. 4,596,588 describing methods for separating hydrocarbon gases using a two-column system. Many of the methods disclosed in these patents sought to improve the separation technique by either increasing or providing a leaner reflux stream to the distillation column near the top. For example, see U.S. Pat. Nos. 171,964 and 4,278,457. These patents disclose that the separation process may be improved by generating leaner reflux from the feed gas by heat exchange with the overhead vapor stream from a demethanizer column. U.S. Pat. Nos. 4,318,723 and 4,350,511 further teach that the overhead vapor stream should not only be warmed, but also that a portion may be condensed and returned to the distillation column as reflux. In a further modification, U.S. Pat. No. 4,687,499 discloses that the warmed and compressed overhead vapor stream should be further chilled and expanded before return to the demethanizer column as reflux. In a still further variation, U.S. Pat. No. 4,851,020 discloses a process wherein a recycle stream containing liquid is returned to the top of a demethanizer column to improve the ethane recovery in the NGL product. All of these prior art methods attempt to improve the NGL recovery processes by either generating leaner reflux or recycling a portion of the overhead vapor from the demethanizer column.
A significant cost in NGL recovery processes is related to the refrigeration required to chill the inlet gas. Refrigeration for these low temperature recovery processes is commonly provided by external refrigeration systems using ethane or propane as refrigerants. In some applications, mixed refrigerants and cascade refrigeration cycles have been used. Refrigeration has also been provided by turbo expansion or work expansion of the compressed natural gas feed with appropriate heat exchange.
Traditionally, the gas stream is partially condensed at medium to high pressures with the help of either external propane refrigeration, a turboexpander or both. The condensed streams are further processed in a distillation column, e.g., a demethanizer or deethanizer, operated at medium to low pressures to separate the lighter components from the recovered hydrocarbon liquids. Turboexpander technology has been widely used in the last 30 years to achieve high ethane and propane recoveries in the NGL for leaner gas. For richer gas containing significant quantities of heavy hydrocarbons, a combined process of turboexpander and external propane refrigeration is the most efficient approach.
As can be seen from the foregoing description, the prior art has long sought methods for improving the efficiency and economy of processes for separating and recovering natural gas liquids from natural gas. Accordingly, there has been a long-felt but unfulfilled need for more efficient, more economical methods for performing this separation. The present invention provides significant improvements in efficiency and economy, thus solving those needs.